Host-Parasite Relationships PDF

HOST PARASITE RELATIONSHIPS Dr/Sarah Samy Assistant Lecturer of Microbiology&Immunology Faculty of Medicine Suez University  Host A host is a larger organism that harbors a smaller organism; whether a parasitic, a mutualist...

HOST PARASITE RELATIONSHIPS Dr/Sarah Samy Assistant Lecturer of Microbiology&Immunology Faculty of Medicine Suez University  Host A host is a larger organism that harbors a smaller organism; whether a parasitic, a mutualistic, or a commensalist guest i.e organism from which a parasite obtains its nutrition and/or shelter.  Parasite Living on/in the host → damages → can cause disease Obtaining necessities of life from the host Types: Non living: unique protein (prion), nucleic acid (viroid) Non living/living: virus prokaryote: bacterium Eukaryotes: microscopic fungi,protozoa,helmiths  Introduction The host and parasite are in dynamic interaction, the outcome of which depends upon the properties of the parasite and of the host. The parasite has its determinants of virulence that allow it to invade and damage the host and to resist the defenses of the host. The host has various degrees of resistance to the parasite in the form of the host defenses.  Host Defenses Pathogen Defenses A microorganism will not be Pathogens contain virulence able to invade unless it factors that promote disease overcomes an animal’s or formation and provide the individual’s host defenses. opportunity for a microbe to infect and cause disease. Specific host defenses may include: The greater the virulence, the  Skin and mucosal secretions more likely disease will occur.  Non-specific local responses Such factors include: (e.g., pH)  Ability of a pathogen to  Non-specific inflammatory adhere to a host responses  Ability of a pathogen to  Specific immune responses colonize (overcome) a host (e.g., lymphocytes)  Ability of a pathogen to evade host defenses INFECTIOUS DISEASE : is disease caused by pathogenic microorganisms. Pathogenic = disease causing Virulence = degree or intensity of pathogenicity Disease = abnormal state, deviation from a state of wellness or health Contamination means that microorganisms are present. Infection, when parasitic microorganisms increase in number either within or on the body of the host  Host-Pathogen Interactions This interaction between the host and the organism is referred to as symbiosis. There are three forms of symbiotic relationships that can occur at an anatomical level 1. Mutualism: Both members benefit from the relationship. For humans, one classic mutualistic association is that of the lactic acid bacteria that live on the vaginal epithelium of a woman. The bacteria are provided habitat with a constant temperature and supply of nutrients (glycogen) in exchange for the production of lactic acid, which protects the vagina from colonization and disease caused by yeast and other potentially harmful microbes. 2. Commensalism: is a relationship in which one member benefits, and the other one neither benefits nor is harmed. Examples of this include microorganisms that make up the normal flora that inhabit the eyes. These organisms thrive on secretions and dead cells, but do not affect the host. 3. Parasitism: Are those relationships in which one member benefits, and the other one is harmed in some way. For example, parasites use the gastrointestinal tract of a human or animal as an environment in which to reproduce.  Bacterial Pathogenesis A pathogen is a microorganism (or virus) that is able to produce disease. Pathogenicity is the ability of a microorganism to cause disease in another organism The Body’s Normal Flora: The body contains two types of normal flora: 1) resident flora (survive for extended periods), and 2) transient flora (temporary). Normal flora help to provide 1. Defenses against invading pathogens by covering adherence sites, producing compounds toxic to other organisms. 2. preventing pathogens from consuming available nutrients. 3. Production of nutrients- Vitamins B and K in the intestines by E. coli Mechanisms of Pathogenesis  Opportunistic infection Defined as infection caused by non- pathogenic microorganisms (bacterial, viral, fungal, or protozoan) which become pathogenic when the immune system is systemically or locally impaired by an unrelated disease such as cancer, diabetes, HIV infection, and other immunodeficiencies or immunosuppression therapy As Candida albicans: Associated with oral thrush and gastrointestinal infection  Virulence Factors: In order for a bacterium to be virulent, it must have capabilities that allow it to infect a host as: 1. Adherence and Colonization 2. Invasion 3. Toxins 4. Enzymes 5. capsules 6. Pili  1-Adherence and Colonization Adherence is an essential step in bacterial pathogenesis or infection, required for colonizing a new host Most fimbria of gram-negative bacteria function as adhesins  Biofilm Many normal flora bacteria produce a capsular polysaccharide matrix or glycocalyx to form a biofilm on host tissue. Biofilms are groups of bacteria attached to a surface and enclosed in a common secreted adhesive matrix, typically polysaccharide in nature These bacteria are better able to: 1. resist attack by antibiotics; 2. trap nutrients for bacterial growth and remain in a favorable niche; 3. adhere to environmental surfaces and resist flushing.  2-Invasion enabling bacterium to invade eukaryotic cells facilitate entry at mucosal surfaces. Some of these invasive bacteria (such as Rickettsia and Chlamydia species) are obligate intracellular pathogens. Bacteria have evolved numerous structural and metabolic virulence factors that enhance their survival rate in the host. Capsule formation has long been recognized as a protective mechanism for bacteria.Encapsulated strains of many bacteria (e.g., pneumococci) are more virulent and more resistant to phagocytosis and intracellular killing than are none capsulated strains.  3-Toxins Bacterial toxins are typically classified under two major categories: exotoxins or endotoxins. Exotoxins are immediately released into the surrounding environment whereas endotoxins are not released until the bacteria is killed by the immune system  Exotoxins Endotoxins Mostly Gram positive bacteria Produced by and also Gram negative Gram negative bacteria. bacteria. Lipopolysaccharide-protein Protein (polypeptide) Chemical Nature complexes complexes Lipid A Located on extrachromosomal Located on chromosomal Chromosomal Location genes (e.g. plasmids). genes. Stability to heat Heat labile (60-80°C) Heat stable (250°C) Toxicity Highly toxic Moderately toxiC Immunogenicity Highly immunogenic. Weakly immunogenic. Possible (On treatment with formalin). eg. For the Conversion to Toxoids Not possible prevention of diphtheria, botulism, and tetanus. Often fatal in larger quantities fatal Do not prove fatal  Infection An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce How Do Infections Occur? 1-source of infection: People, environments, Dust 2-Susceptible Person 3- Transmission:  The Chain of Infection The spread of an infection within a community is described as a “chain,” several interconnected steps that describe how a pathogen moves about. The spread of infection can be described as a chain with six links: 1. Infectious agent (pathogen) 2. Reservoir (the normal location of the pathogen) 3. Portal of exit from the reservoir 4. Mode of transmission 5. Portal of entry into a host 6. Susceptible host  Reservoir The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment  Human reservoirs may or may not show the effects of illness. As noted earlier, a carrier is a person with inapparent infection who is capable of transmitting the pathogen to others. Asymptomatic or passive or healthy carriers are those who never experience symptoms despite being infected. Incubatory carriers are those who can transmit the agent during the incubation period before clinical illness begins. Convalescent carriers are those who have recovered from their illness but remain capable of transmitting to others. Chronic carriers are those who continue to harbor a pathogen such as hepatitis B virus or Salmonella Typhi  Animal reservoirs Humans are also subject to diseases that have animal reservoirs. Many of these diseases are transmitted from animal to animal, with humans as incidental hosts. The term zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans.  Environmental reservoirs Plants, soil, and water in the environment are also reservoirs for some infectious agents. Many fungal agents, such as those that cause histoplasmosis, live and multiply in the soil.  Mode of transmission 1. Direct contact: includes host-to-host contact, such as through kissing or sexual intercourse, where one person might come in contact with another person’s skin or body fluids. 2.Indirect contact: involves the transfer of the infectious agent through some type of intermediary, such as a contaminated object. The pathogen might be deposited on an inanimate object, called a fomite, which is then used by another person. This could include a shared toy or commonly-touched surface, like a doorknob or computer keyboard. 3. Droplet transmission: infections transmitted by particles larger than 5 µm These microorganisms are relatively large and travel only short distances (up to 2 metres). For this reason there may be both Droplet and Contact Precautions required at the same time. Examples of microorganisms that are spread by droplet transmission are: influenza, Coronavirus, respiratory syncytial virus (RSV 4. Airborne transmission: infections transmitted by particles 5 µm or smaller Microoganisms carried in this manner remain suspended in the air for long periods there is risk that all the air in a room may be contaminated. examples of microorganisms that are transmitted by the airborne route are: M. tuberculosis, rubeola, varicella 5. Fecal-oral transmission: an infected host is shedding the pathogen in their feces which contaminate food or water that is consumed by the next host. 6. Vector borne transmission: occurs when an arthropod vector, such as mosquitoes, flies, ticks, are involves in the transmission. 7.Food and water transmission 8.Spread through body fluids or blood such as hepatitis B&C, HIV  References https://microbenotes.com/host-parasite- interactions/ https://www2.nau.edu/~fpm/bio205/host_para. html http://textbookofbacteriology.net/NHPR.html http://agrilife.org/vetmed/files/2012/10/OH_4_4 _sample_lesson.pdf https://bio.libretexts.org/Bookshelves/Microbiol ogy/Book%3A_Microbiology_(Bruslind)/21%3A_ Bacterial_Pathogenicity